The present invention relates generally to a voltage supervisory circuit for monitoring input voltage levels. More particularly, the present invention relates to a circuit for supervising at least one input voltage of a multi-rail power supply, and generating a power-good signal responsive to all supervised input voltages being greater than a predetermined threshold voltage.
Developers of products which are voltage sensitive, such as computers, can face many problems. One problem is that certain computer components require a minimum threshold voltage to achieve predictable performance. For example, a voltage supply could corrupt nonvolatile memory due to the supply voltage being low relative to the minimum threshold voltage, as may occur during power-up and during a brown out. Additionally, a central processing unit could behave erratically and furnish erroneous responses during a fluctuating situation voltage such as a power dropout. Thus, it is important for developers of such products to know when power is considered good, i.e., above a certain threshold voltage (a "power-good" situation).
While monitoring a power-good situation when there is only one supply voltage is difficult, the difficulty increases when the product being powered requires multiple voltage levels from a single power supply or from various individual power supplies, such as many modem products require today. In addition to a traditional 5V power supply, new products are beginning to utilize 3.3V, 2.5V, and 1.9V supplies. Previous solutions to address power-up and under voltage problems have been inadequate. Some developers have ignored the problem entirely and have implemented no protection, thereby exposing themselves to the erratic behavior described above. Other developers have utilized circuitry that detects low voltage conditions only on a primary power supply rail. This solution is inadequate because it ignores the other power supply rail voltages. (While the application refers to supply rails and rail voltages, it is contemplated that power buses may also be utilized). Similarly, while some developers have implemented circuitry that monitors all the power supply rails, additional problems exist.
One problem developers face is the lack of availability of a single circuit that functions to provide multiple rail voltage detection. The few monolithic integrated circuit (IC) devices in existence which are capable of detecting multiple voltages are expensive. Further, generally the ICs that exist do not support all possible invalid power scenarios. They either support under voltage, reset, or dropout detection, but typically not all three. In this regard, under voltage occurs when the power shifts from good, i.e., above the minimum threshold voltage, to bad, which is below the minimum threshold voltage. Such a situation transpires, for example, during a brown out, or when the power is turned off and lingering voltage on the rail exists due to inherent capacitance on the rail. Reset ensues when voltage levels proceed from bad to good such as during power-up. Dropout occurs when voltage levels move from good to bad to good, all within a short time frame of about 10-20 ms.
Other problems include the fact that existing ICs do not allow for a time delay, preferably at least about 200 ms, of acceptable power levels, and if they do, the period of delay cannot be changed from the original arbitrary setting established by the manufacturer. Likewise, the manufacturer typically sets the threshold voltage which cannot be changed. Moreover, while some solutions properly apply a delay to the power-good signal when power first comes up, they fail to properly delay the signal during the quick dropout cycle. Similarly, current known ICs fail to deactivate the time delay immediately upon dropout.
Accordingly, in response to the problems discussed above, a primary object of the present invention is to provide an improved apparatus for supervising an input voltage of a multi-rail power supply and generating a power-good signal responsive to all supervised input voltages being greater than their respective predetermined threshold voltages.
Another object of the present invention is to provide a relatively inexpensive, single improved apparatus that functions to allow multiple rail voltage detection.
A further object of the present invention is provide such an improved voltage detection apparatus that supports all possible invalid power scenarios.
An additional object of the present invention is to provide an improved apparatus that allows for an adjustable time delay and threshold voltage.
Yet another object of the present invention is to provide such an improved apparatus which cancels the power-good signal immediately when the supply voltage falls below the threshold voltage, and to incorporate the proper delay before re-establishing the power-good signal after the supply voltages achieve the threshold voltage.